The present invention relates to the field of materials, and, more particularly, to the field of semiconductor materials having certain dielectric properties.
Semiconductor devices are widely used in many applications. For example, an early version of an integrated circuit fingerprint sensor produced by Harris Corporation was based upon sensing an electric field between the sensor and the ridges and valleys of a fingerprint of a user. Such a sensor may be extremely accurate in generating an image of the ridges and valleys of the fingerprint.
The fingerprint sensor relied on direct contact between the finger of the user and the integrated circuit. Such direct contact can lead to several difficulties with regards to the long term reliability of the sensor. For example, sodium ions from perspiration may migrate through the relatively thin outer passivation layer or layers and adversely affect the semiconductor material of the sensor. Solvents for cleaning the sensing surface may also damage the integrated circuit.
Typical passivation layers for integrated circuit technologies are relatively thin, since the IC is usually protected by an overall body of molded encapsulating material. The encapsulating material provides both mechanical protection, as well as protection from contamination of the semiconductor material. Unfortunately, in an application such as the electric field fingerprint sensor, the IC die itself must be exposed to direct contact. Moreover, simply increasing the thickness of passivation or protective coatings may reduce the quality of the fingerprint image. This is so because the electric fields of the individual pixel elements of the sensor tend to curve or defocus as the spacing between the elements and the finger is increased.
U.S. Pat. No. 4,353,056 to Tsikos discloses an early approach to sensing a live fingerprint. In particular, the patent discloses an array of extremely small capacitors located in a plane parallel to the sensing surface of the device. When a finger touches the sensing surface and deforms the surface, a voltage distribution in a series connection of the capacitors may change. Unfortunately, the resilient materials required for the sensor may suffer from long term reliability problems. Moreover, noise and stray capacitances may adversely affect the plurality of relatively small and closely spaced capacitors.
U.S. Pat. No. 5,325,442 to Knapp discloses another fingerprint sensor and which includes a plurality of sensing electrodes. A capacitor is effectively formed by each sensing electrode in combination with the respective overlying portion of the finger surface which, in turn, is at ground potential. The sensor may be fabricated using semiconductor wafer and integrated circuit technology. The dielectric material upon which the finger is placed may be provided by silicon nitride or a polyimide which may be provided as a continuous layer over an array of sensing electrodes.
Unfortunately, such conventional semiconductor related materials and their relative thinness may not be sufficient for direct contact by the finger of a user. Moreover, increasing the thickness of any coating layer may adversely affect the image accuracy or resolution. Accordingly, at present the designer needs to sacrifice robustness of the IC fingerprint sensor to obtain sufficient accuracy in the image produced.
In view of the foregoing background, it is therefore an object of the present invention to provide a method for making a dielectric layer, such as for integrated circuits, that is relatively thick, yet which has reduced defocusing of an electric field passing therethrough.
It is another object of the present invention to provide integrated circuits and layers having a dielectric layer with certain desirable dielectric properties.
These and other objects, features, and advantages in accordance with the present invention are provided by a method for making an anisotropic dielectric layer comprising the steps of: forming a fluid layer comprising a plurality of magnetizable particles in a fluid capable of solidifying to fix the configuration of the magnetizable particles in a dielectric matrix; aligning the magnetizable particles of the fluid layer in a predetermined configuration by applying a magnetic field thereto; and fixing the aligned magnetizable particles in the predetermined configuration within the dielectric matrix by solidifying the fluid to thereby make the anisotropic dielectric layer. In one particularly advantageous application, the fluid layer is coated onto a surface portion of an integrated circuit, such as a fingerprint sensor, to provide mechanical protection without effecting the image quality or resolution. In addition, the step of aligning for certain devices preferably comprises aligning the magnetizable particles in a predetermined configuration so that an impedance in a direction perpendicular to the anisotropic dielectric layer is less than an impedance in a direction parallel to the anisotropic dielectric layer.
The magnetizable particles may be mixed in a curable polymer fluid, and the step of fixing the aligned magnetizable particles may comprise curing the curable polymer fluid, such as by applying heat or radiation. The magnetizable particles may be generally spherical having diameters in a range of about 1 to 3 microns. The magnetizable particles may also be generally elongate.
The method may also include the step of controlling a viscosity of the fluid by incorporating dielectric particles in the fluid. For example, the size and/or concentration of the dielectric particles may be controlled in the curable polymer fluid. The dielectric particles may also reduce lateral coupling of the magnetizable particles.
The step of aligning the magnetizable particles preferably comprises applying a substantially uniform magnetic field to the fluid layer, such as achieved by positioning a pair of opposing magnets adjacent opposite sides of the fluid layer and extending laterally outwardly beyond edges thereof.
An integrated circuit including the anisotropic layer preferably also comprises a substrate, and a semiconductor layer adjacent the substrate. The anisotropic dielectric layer is preferably adjacent the semiconductor layer, and the anisotropic dielectric layer preferably comprises a dielectric matrix and a plurality of aligned magnetizable particles therein. The magnetizable particles may be aligned in a predetermined direction so that the anisotropic dielectric layer has an impedance in a direction perpendicular to a surface being less than an impedance in a parallel direction. In addition, the semiconductor layer may include means for passing an electric field through the anisotropic dielectric layer, such as for sensing applications.
Another aspect of the invention relates to the dielectric layer. The dielectric layer preferably comprises a plurality of aligned magnetizable particles fixed in a dielectric matrix, such as to provide an impedance in a first direction which is less than an impedance in a second direction transverse to the first direction.